In a hot strip mill, relatively wide steel slabs are rolled into very thin strips. Typically, hot strip mills operate in three stages: 1) the roughing stage, 2) the finishing stage, and 3) the coiling stage. In the roughing stage the slab is directed to a roughing mill where thickness reduction takes place. For example, it is typical for a slab having a thickness of between 8 and 10 inches to be reduced to a thickness of 11/4 inches. In the finishing stage the reduced slab, hereinafter referred to as a strip, is directed to a finishing mill where it is reduced to a strip approximately 0.500-0.050 inches thick. From the finishing mill, the strips are coiled for convenience of handling and further processing. Traditionally, finishing mills consist of approximately four to seven closely spaced stands.
Heretofore, hot strip mills have been plagued by rapid and excessive wear of the work rolls. This condition is triggered when abrasive iron oxide layers form on steel strips and the strips are pressed by the work rolls. When the strips contact the work rolls, the iron oxide grinds against the work roll surface thus causing the rolls to become excessively worn and to deteriorate prematurely. In addition, the growth of the oxide can be so great as to fracture during rolling creating surface defects called "heat pattern", "salt and pepper" and "roll wear" scale--three common varieties of rolled in scale. These defects combined with the total oxide thickness affect the removal of the oxides by acid pickling on subsequent processing units. (There are several different types of iron oxide layers that form on steel strips. When discussed collectively hereinbelow, they will be referred to as iron oxide).
Iron oxide formation is a function of temperature and time. That is, the longer the strip is exposed to high temperatures, the higher the rate and volume of iron oxide formation. The temperature in a typical finishing mill ranges between 1500.degree. F. and 1900.degree. F. In that temperature range, steel oxidizes rapidly; and, therefore, in a relatively short period of time substantial amounts of iron oxide are formed on the strip prior to its entrance into the finishing mill.
A known technique for removing iron oxide is to spray the strip with pressurized liquid. This technique is sometimes referred to as "descaling" and it is discussed in more detail in U.S. Pat. No. 3,766,763 issued to Cofer et al. and U.S. Pat. No. 4,043,166 issued to Leroy. While the descaling technique is successful in removing iron oxide from the strips, it does not address the fundamental problem of iron oxide formation. Hence, although iron oxide is removed from the strips during descaling, iron oxide is reformed on the strips as they pass through the mills and are exposed to high temperatures. Accordingly, the descaling technique will not adequately protect against wear and deterioration of the work rolls of a mill, nor can it reduce the total oxide formations that cause rolled in scale defects or influence the removal of oxide during subsequent processing.